Laboratory studies in pollution chemistry, occupational health hazards and toxicology require the continuous generation of accurately known quantities of reactive gases in dilute concentration. Such reactive gases are needed for the calibration of measuring instruments, to evaluate analytical methodology, to carry out toxicity investigations and in physico-chemical studies on the rate of mechanisms of pollutant formation.
The reactive nature of the gases makes standard static methods for preparation of the gases unsatisfactory so that a dynamic method is needed where the required samples are prepared immediately before use. U.S. Pat. No. 3,740,195 discloses a continuous reactive gas generator which uses a moving metal ribbon coated with a pyrolyzable salt. However, this device uses a non-volatile salt and would not work with a volatile organic liquid which would be vaporized before introduction into the pyrolysis zone. In addition, pyrolysis of a solid often gives variable and irreproducible results. Furthermore, the use of a salt as a parent compound is not versatile since the types of thermally stable reactive gases which can be generated from salts if quite limited.
The paper "Pyrolysis Generation of Dilute Concentration of Sulphur Dioxide" in Analytical Chemistry, Vol. 46, page 933, June 1974 discloses a sample tube in combination with a pyrolyzing tube. The sample tube contains the pyrolyzable parent compound and is enclosed in an oven. Carrier gas is passed over the compound in the sample tube. This approach was selected in preference to the use of a diffusion cell for the following reasons: (a) The pyrolytic decomposition process must be carried out in the total absence of air. In the usual diffusion cells this is not achievable because even the slightest leak will throw off the results. (b) Standard diffusion cells are monolithic (no joints or valves, etc.) and are made of glass. Such a construction is very difficult to work with.
The publication "A Simple Technique for the Generation of Dilute Mixtures of Pollutant Gases", Journal of Research of the National Bureau of Standards, Vol. 78A, No. 2, page 157, March-April, 1974, discloses a simple quantitative means of generating dilute mixtures of formaldehyde, acetaldehyde and acrolein by the pyrolysis of compounds the decomposition of which produce equal numbers of reactive and stable molecules. However, a pulse technique is used and this limits its range of potential applications. For toxicity and physico-chemical investigations continuous generation is essential; even for calibration purposes continuous generation is more effective because it more closely simulates the situation being studied.
The prior art shows methods for the production of small concentrations of reactants including precision flow dilution systems and permeation tubes. But the accuracy of these methods depends critically on the skill in preparing the sample or setting reaction conditions. Further, they do not work well over a wide range of concentrations.
Thus, there continues to exist in the art the need for means for the continuous generation of accurately known quantities of reactive gases in dilute concentrations, and the problem is particularly difficult because the desired gases are reactive yet must be obtained in dilute concentrations.